This invention relates to an antenna generating plural beams of radiation and, more particularly, to an antenna having front and rear antenna dish-shaped reflectors illuminated respectively by separate offset front and rear feeds, wherein the front reflector is transparent to radiation to be reflected by the rear reflector, the antenna having a compactness of size afforded by maximizing design flexibility.
Communications satellites encircling the earth may carry various antennas for forming beams of radiation for up-link received signals and down-link transmitted signals. The beams may be directed to one or more regions on the earth's surface, depending on the mission of the satellite. It is desirable to minimize the weight of an antenna system so as to allow the satellite to carry a larger payload. It is also highly desirable to minimize the size of the antenna.
One form of satellite antenna system comprises two antennas mounted within a single structure and providing for two separate beams for carrying two separate signals to different locations on the earth's surface. A support of the antenna system holds two antenna reflectors in tandem, namely, a rear reflector substantially behind a front reflector. The support also holds a front feed for illuminating the front reflector to produce a front beam, and a rear feed for illuminating the rear reflector to produce a rear beam. In one form of construction of antenna system, the two feeds generate beams of cross-polarized linear polarizations, such as horizontal and vertical polarizations, and the front reflector is reflective to radiation at one of the two polarizations while being transmissive to the radiation to be reflected by the rear reflector.
A problem arises with the foregoing type of antenna system in that the front reflector is not totally transparent to the rear-feed radiation, and reflects the rear-feed radiation as an interfering beam. Degradation of antenna performance occurs in the event that the interfering beam falls within the region of coverage of the front beam and interferes with the front beam.
A further problem arises with the foregoing type of antenna system in that, due to the offset positions of the two feeds, there are rays from the rear feed which pass through the front reflector to illuminate the rear reflector while other rays from the rear feed bypass the front reflector to illuminate directly the rear reflector. The front reflector, while being classified as being transparent to the radiation of the rear feed, does introduce a variation in direction of propagation and intensity as compared to the rays which bypass the front reflector. Thus, there is a partial shading of the rear reflector by the front reflector from rays of the rear feed. The resulting lack of uniformity in the illumination of the rear reflector introduces a degradation in the radiation pattern of the beam produced by the rear reflector.